1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor film having a crystal structure by using a laser crystallization technique and a semiconductor device such as a thin film transistor (hereinafter, TFT) including the semiconductor film. More specifically, the present invention relates to a method for manufacturing a crystalline semiconductor film having crystal grains in which crystal grain boundaries are almost parallel to each other and the widths of the crystal grains are controlled, and a semiconductor device including the crystalline semiconductor film.
2. Description of the Related Art
In recent years, a laser crystallization technique of irradiating an amorphous semiconductor film formed over a glass substrate with a laser beam to form a semiconductor film having a crystal structure (hereinafter referred to as a crystalline semiconductor film) has been widely researched, and a large number of suggestions are given.
Since a crystalline semiconductor film has higher mobility than an amorphous semiconductor film, the crystalline semiconductor film is utilized for TFTs. Consequently, TFTs using the crystalline semiconductor film are used for an active matrix type liquid crystal display device, an organic EL display device, and the like in which, over a glass substrate, TFTs are formed for a pixel portion or for a pixel portion and a driver circuit.
As a crystallization method, other than laser crystallization, there are a thermal annealing method using an annealing furnace and a rapid thermal annealing (RTA) method. However, when the laser crystallization is used, heat is absorbed only by the semiconductor film and crystallization can be performed without increasing temperature of the substrate too much. Therefore, a substance such as glass, plastic, or the like that has a low melting point can be used as the substrate. As a result, a glass substrate which is inexpensive and is easily processed even in a large area can be used; thus, production efficiency can be drastically improved by the laser crystallization.
Attention has been focused on excellent characteristics of such a crystallization method and a semiconductor film has been tried to be manufactured by laser crystallization. Consequently, the development of many techniques has succeeded. Among the successful techniques, there is a technique of obtaining a crystalline semiconductor film having excellent semiconductor characteristics, particularly high mobility, by the steps of irradiating an amorphous semiconductor film in which the concentration of oxygen, nitrogen, and carbon is reduced with a laser beam and subjecting the amorphous semiconductor film to a melting step and a recrystallization step (Patent Document 1: Japanese Published Patent Application No. H5-299339). In this technique, it is suggested that in laser beam irradiation, a protective film is preferably formed over the amorphous semiconductor film, and by the placement of the protective film, an impurity can be prevented from mixing into the semiconductor film.